The best time to do 4D ultrasound is 22-26 weeks of pregnancy, when the main organs of the fetus have developed, the size of the fetus and the amount of amniotic fluid are moderate, the space for intrauterine activities is relatively large, the skeletal echoes are less influential, and the examination images are clearer, which facilitates the screening of fetal malformations. 4D ultrasound has the effect of real-time dynamic observation on the basis of 3D ultrasound, which can burn the whole examination process into a CD to record every move of the fetus, and more importantly, it can observe the growth and development of the fetus in the uterus and screen for malformations. It can make more accurate judgments about the fetal face, digestive system, urinary system, brain development and heart development, provide accurate scientific basis for early diagnosis of fetal congenital anomalies and congenital heart diseases, and terminate pregnancy in time if serious lethal anomalies are detected. It is not necessary to hold urine or fast when doing 4D examination, instead, it is recommended to examine after eating to avoid the reduced intrauterine activity of the fetus in the hungry state, which will affect the examination effect.